[libcxx] [test] Coverage improvement for std::future_error::what().

[amakc11]

Summary

The call of this method is inside the LIBCPP_ASSERT macro that removes the call when running tests against other libraries. This patch moves the call out of the macro to achieve correct coverage. Also, the path adds the following library-independent checks:

1. each string returned by this method is not empty;
2. all returned strings are distinct.

libcxx/test/std/thread/futures/futures.future_error/what.pass.cpp

 
 // <future>
 
 // class future_error
 
 // const char* what() const throw();
 
 #include <future>
-#include <cstring>
+#include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
+    std::string s[4];
+
     {
         std::future_error f(std::make_error_code(std::future_errc::broken_promise));
-        LIBCPP_ASSERT(std::strcmp(f.what(), "The associated promise has been destructed prior "
-                      "to the associated state becoming ready.") == 0);
+        s[0] = std::string(f.what());
+        LIBCPP_ASSERT(s[0] == "The associated promise has been destructed prior "
+                      "to the associated state becoming ready.");
     }
     {
         std::future_error f(std::make_error_code(std::future_errc::future_already_retrieved));
-        LIBCPP_ASSERT(std::strcmp(f.what(), "The future has already been retrieved from "
-                      "the promise or packaged_task.") == 0);
+        s[1] = std::string(f.what());
+        LIBCPP_ASSERT(s[1] == "The future has already been retrieved from "
+                      "the promise or packaged_task.");
     }
     {
         std::future_error f(std::make_error_code(std::future_errc::promise_already_satisfied));
-        LIBCPP_ASSERT(std::strcmp(f.what(), "The state of the promise has already been set.") == 0);
+        s[2] = std::string(f.what());
+        LIBCPP_ASSERT(s[2] == "The state of the promise has already been set.");
     }
     {
         std::future_error f(std::make_error_code(std::future_errc::no_state));
-        LIBCPP_ASSERT(std::strcmp(f.what(), "Operation not permitted on an object without "
-                      "an associated state.") == 0);
+        s[3] = std::string(f.what());
+        LIBCPP_ASSERT(s[3] == "Operation not permitted on an object without "
+                      "an associated state.");
+    }
+
+    for ( int i = 0; i < 4; i++ )
+    {
+        assert(s[i].length() != 0);
+        for ( int j = i+1; j < 4; j++ )
+        {
+            assert(s[i].compare(s[j]) != 0);
+        }
     }
 
   return 0;
 }